Flirty Girlfriend Issues?

My girlfriend hangs out with my friends, I never hang out with her friends. Whenever she talks to my friends who are supposedly her best friends now, she gets really close to them and touches them like in their arm or stomach and giggles and flirts like crazy. When I confronted her about this, she said that ';They have nothing on you'; and that I'm being jealous. If I were in my friends' positions though, I'd seriously think I'd be able to get in her pants in 2 months. She does this to guys that I'm not friends with too. It bothers the hell out of me and looks terrible on her part. People have come up to me and have said ';What the hell is your girlfriend doing?'; She'll leave me standing by myself after school to go talk to MY friends and won't offer for me to join her. Mid conversation she'll go ';Hold on, one sec'; and just walk away and come back 5 minutes later. When I ask her what she talked about it's always something retarded and unnecessary. I think I should break up with her. She knows it bothers me to no end and continues as if she's not doing anything and claims ';That's just how she is';. She wasn't like this before I was dating her. What should I do?Flirty Girlfriend Issues?
in my opinion, i think you should leave her . if she's treating you like this, show her how much pain she has caused you . if she denys doing any of tht stuff you know the type of girl she is and according to you, so does everyone else . (people have come up to me...) anyways, either way if yo dump her, she'll either realize what shes missing then come back to you, or you'll see what she truely wanted .. which was probably just a guy to get her more friends ..